Neutron source



United States Patent 3,183,356 NEUTRON SOURCE Julian H. Cherubini,Quincy, Mass, assignor to High Voltage Engineering Corporation,Burlington, Mass, a corporation of Massachusetts N0 Drawing. Filed July30, 1962, Ser. No. 213,130 1 Claim. (Cl. 250-845) This invention relatesto neutron sources and more particularly to a novel lithium tritidetarget and to an improved method of preparation thereof.

Neutron sources of the type in which a neutron emissive target isbombarded by a high energy deuteron beam or the like often requiretargets having an abundant yield of high energy neutrons. It isdesirable that such targets have a reasonably long life. The currentstate of the art reveals no target which effectively meets all of theabove criteria. Long life targets such as those made of beryllium ordeuterated wax yield relatively low energy neutrons. Polonium-berylliumand lithium-beryllium targets are limited in their neutron output. Inorder to obtain both high energy neutrons and high neutron outputs, thetritium target has often been resorted to. However, conventional tritiumtargets do not last very long if the accelorator producing the highenergy deuteron beam is operated at full power. Typically, these targetsconsist of titanium or zirconium into which tritium gas has beenabsorbed. The neutron yield decreases all too readily when the target isbombarded by a deuteron beam and the resulting rapid targetdecomposition necessitates frequent replacement thereof. A furtherproblem associated with prior art neutron source targets is that varioustarget materials react with air and moisture to produce a metalhydroxide and a hydrogen gas. Thus, when not in the proper environmentsuch as a vacuum system, the target may spontaneously decompose.

Accordingly, it is a principal object of this invention to provide, in aneutron source, a new and improved high energy, high yield targetmember.

It is another object of this invention to provide a new and improvedmethod of producing a neutron source target member.

it is another object of this invention to provide a neutron sourcetarget member comprising discrete amounts of lithium tritide dispersedwithin a metal matrix.

It is another object of this invention to provide a long life neutronsource target adapted to produce a copious supply of high energyneutrons.

It is another object of this invention to provide a lithium tritideneutron source target that is impervious to air and moisture.

:It is another object of this invention to provide a practical neutronsource target member having a lower cross sectional stopping area andconsequent higher neutron output than the conventional titanium-tritiumtarget.

It is another object of this invention to provide a neutron sourcetarget member comprising lithium tritide disposed in an aluminum matrixwherein said lithium-tritide is greater than twenty percent by volume ofthe mixture thereof.

These, together with other objects and features of this invention willbecome apparent from the following detailed description of the noveltarget compositions and preferred fabrication method. While specificmaterials and apparatus are referred to herein, it is intended that theybe taken as illustrative of the principles of the invention and not in alimiting sense, the scope of the invention being defined by the appendedclaim.

The present invention comprehends a neutron source 3,183,356 PatentedMay 11, 1965 target member comprising discrete amounts of lithiumtritide dispersed in a metal matrix. It has been determined that thereis considerable advantage in the high energy neutron yield of alithium-tritide target over that of the conventional titanium-tritiumtarget. In addition to having a lower cross sectional stopping area thantitaniumtritium, there is a Li (d, n)Be reaction associated with thelithium-tritide target that produces neutrons at substantially the sameenergy as does the tritium. These unique characteristics have beenrecognized and utilized in combination with other features hereinafterdisclosed to provide a long life target having a neutron outputapproximately three times that of prior art targets of this type.

It is a particular feature of this invention to disperse thelithium-tritide in a metal matrix such as aluminum. The lithium-tritide,being a salt, exhibits very low thermal conductivity. Therefore, inorder to obtain adequate neutron output it is necessary to heat it to ahigh temperature (operate at a high power level). This, however, effectsrapid decomposition of the target by thermal dissociation. Accordingly,the lithium-tritide is dispersed in an aluminum, or other highlyconductive metal, matrix producing a target having a thermalconductivity that is substantially an average of that of the salt andthe metal. The resulting high thermal conductivity target permits themaintaining of as low a temperature as possible, or, alternatively,allows operation at a higher power level at a safe lithium tritidetemperature.

The lithium-tritide target as described above is per se atmosphericallyunstable. That is, it reacts spontaneously with moisture in the air toproduce lithium hydroxide and hydrogen gas. In order to permitconvenient handling and storing of these targets and to prevent theirdegradation when outside of the vacuum system, it is another feature ofthis invention to vapor coat them as a final step of the fabricationprocess. Such a vapor coat of copper or aluminum or the like isolatesthe lithium from atmospheric contact without significantly affecting theetficiency of the target.

The method of fabrication of the subject target comprises the followingsteps. An evacuated chamber such as a bell jar is filled with tritiumgas at a pressure of .approximately 7 p.s.i.g. Melted lithium is thenintroduced therein whereby a salt forming reaction takes places betweenthe lithium and the tritium gas. The salt thus formed is then ground toform a powder of less than 170 mesh size (U.S. standard sieve). Aluminumpowder of substantially the same or greater mesh size is then mixedtherewith. The proportion by volume of lithium tritide salt to aluminummay range from 20% lithium tritide/ aluminum to 80% lithium tritide/20%aluminum by volume depending upon the proposed use of the target.Targets having less than 20% lithium tritide are limited by theirneutron output while those having more than 80% lithium tritide arelimited by fabrication parameters and by decreased thermal conductivity.The mixture is then compacted into a foil through a cold pressingprocess or through a modified-evacuated pack rolling technique to athickness of the order of one or two mils and to a density exceeding 98%of theoretical. Coupons of the desired target size are then ptulched outof the foil. The coupons are then vapor coated with a metal such ascopper or aluminum. This is accomplished by placing the coupons togetherwith a hot metal source in an evacuated container wherein the metalevaporates and condenses on contact with the coupon. Finally the couponthus fabricated is soldered onto a backing disc of high conductivitymetal such as copper.

Having thus described the invention, what is claimed as new and desiredto be secured by Letters Patent is:

A neutron emissive target adapted to bombardment by a high power chargedparticle beam for extended periods of time comprising a couponconsisting of a compacted mixture of powdered lithium tritide andpowdered aluminum, a copper backing plate afiiXed thereto, and a surfacecoating of vaporized aluminum powder, said lithium tritide and saidaluminum being powdered to less than 170 mesh size and compacted to adensity exceeding ninety-eight percent of theoretical, said mixturecontaining not less than twenty percent nor more than eighty percent byvolume of lithium tritide powder and being adapted to References Citedby the Examiner UNITED STATES PATENTS Fahrenwald 75-138 Czochralski etal 75-138 Kallmann 31361 X Anderson 176- 10 X Carroll 17610 X Fearon eta1. SIB--61 Goodman 31361 Manning et al. 176l6 Gordon 29--182.5 Thomaset al 313- 61 X Tittle et a1. "a 250-845 15 CARL D. QUARFORTH, PrimaryExaminer.

REUBEN EPSTEI N, Examiner.

